A supersonic plane flies at Mach 3 at an altitude of 20,000 m. A person on the ground sees the plane directly overhead. How much time passes the before she hears the sonic boom?

2. Relevant equations

f=fo(v+-vo/v-+vs)

3. The attempt at a solution

I don't really know how to start up with this one, I was hoping for some help here.

The sonic boom is just a noise produced by the plane, which has to travel 20,000m to reach you.
The answer would be the same if it was a helicopter hovering at 20000m then firing a really loud gun - or with a really big sound system playing AC-DC.